1. Field of the Invention
The present invention relates to the fabrication method of Semiconductor device and specially relates to a light-emitting device which includes an OLED (organic light-emitting device) panel which is formed on a plastic substrate. The invention also relates to an OLED module obtained by mounting an IC that includes a controller to the OLED panel. In this specification, a light-emitting device is used as the generic term for the OLED panel and the OLED module. Also included in the present invention is electronic equipment using the light-emitting device.
2. Description of the Related Art
In recent years, a technique of forming a TFT (Thin film transistor) on a substrate has made great advancement to promote application of TFTs to active matrix display devices. In particular, TFTs using polysilicon have higher field effect mobility (also called mobility) than conventional TFTs that use amorphous silicon and therefore can operate at high speed. This makes it possible to control pixels, which has conventionally been controlled by a driving circuit external to the substrate, by a driving circuit formed on the same substrate on which the pixels are formed.
With various circuits and elements formed on the same substrate, active matrix display devices can have many advantages including lowering of manufacture cost, reduction in display device size, an increase in yield, and improvement in throughput.
An active matrix light-emitting device having an OLED as a self-luminous element (hereinafter simply referred to as light emitting device) is being researched actively. A light-emitting device is also called as an organic EL display (OELD) or an organic light emitting diode (OLED).
Being self-luminous, an OLED does not need back light which is necessary in liquid crystal display devices (LCDs) and is therefore easy to make a thinner device. In addition, a self-luminous OLED has high visibility and no limitation in terms of viewing angle. These are the reasons why light emitting devices using OLEDs are attracting attention as display devices to replace CRTs and LCDs.
An OLED has a layer containing an organic compound (organic light emitting material) that provides luminescence (electroluminescence) when an electric field is applied (the layer is hereinafter referred to as organic light emitting layer), in addition to an anode layer and a cathode layer. Luminescence obtained from organic compounds is classified into light emission upon return to the base state from singlet excitation (fluorescence) and light emission upon return to the base state from triplet excitation (phosphorescence). A light-emitting device according to the present invention can use one or both types of light emission.
In this specification, all the layers that are provided between an anode and a cathode of an OLED together make an organic light-emitting layer. Specifically, an organic light-emitting layer includes a light-emitting layer, a hole injection layer, an electron injection layer, a hole transporting layer, an electron transporting layer, etc. A basic structure of an OLED is a laminate of an anode, a light-emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light-emitting layer, and a cathode layered in this order, a laminate of an anode, a hole injection layer, a light emitting layer, an electron transporting layer, and a cathode layered in this order, or the like.
Various applications of such light-emitting device are expected. In particular, applications to portable equipment are attracting attention because the light-emitting device is thin and accordingly is useful in reducing the weight. This has prompted attempts to form an OLED on a flexible plastic film.
A light-emitting device in which an OLED is formed on a flexible substrate such as a plastic film is thin and light-weight and, moreover, applicable to a curved display or show window, etc. Therefore, the use thereof is not limited to portable equipment and its application range is very wide.
However, plastic substrates in general are well transmissive of moisture and oxygen, which accelerate degradation of organic light emitting layers. Therefore plastic substrates often shorten the lifetime of light-emitting devices. This is solved in prior art by placing an insulating film such as a silicon nitride film or a silicon oxynitride film between a plastic substrate and an OLED to prevent moisture and oxygen from entering an organic light emitting layer.
Plastic film substrates in general are also weak against heat and are easily deformed if the insulating film such as a silicon nitride film or a silicon oxynitride film is formed at a temperature that is too high. On the other hand, if the temperature at which the insulating film is formed is too low, the quality of the film is reduced and the film cannot prevent transmission of moisture and oxygen satisfactorily.
When the insulating film such as a silicon nitride film or a silicon oxynitride film is increased in thickness in order to prevent transmission of moisture and oxygen, the internal stress is increased to likely cause a crack (fissure). The thick insulating film makes the substrate weak against cracking when the substrate is bent.